The metals plating and finishing industry has been moving to a "zero-discharge" basis for disposal of metal-bearing wastewater streams. Progressively stricter regulatory criteria have forced industry to reduce drastically the residual metal contents in wastewater discharges and, in some instances at least, allowable residuals are approaching the limit of detection afforded by available analytical techniques. Further, the economic benefit of "zero-discharge" for recycle of clean and wastewater effluents to process applications and metals recovery is an attractive goal, particularly for reduction of water costs and sewer use charges.
The ability of conventional wastewater treatment methods to achieve the low levels of residual metals required by the higher standards for wastewater purity in many cases is marginal. In addition, most conventional methods of heavy metals removal produce copious amounts of sludge, usually classified a priori as a hazardous material. Recent legislation has made the disposal of sludge material extremely difficult and expensive and no near term solution to the sludge disposal problem seems apparent.
Because of these problems, industry in general, and the metal plating and finishing segments in particular, have been forced to consider alternative methods for heavy metals removal from wastewater streams. The major characteristics needed in heavy metals removal from wastewater streams are: ability to reduce residual metal contents to extremely low levels (ultimately to the parts-per-billion range); production of minimal amounts of sludge; economic operation; production of an effluent suitable for recycle to process operations; and ability for maximum retrofit into existing installations.
One of the more promising new alternative approaches that possesses the potential of fulfilling to a significant degree these desirable requirements for treating metallic-bearing wastewaters is xanthate technology. A patent to John Hanway, U.S. Pat. No. 4,166,032, discloses the use of cellulose xanthate for heavy metals removal from wastewater streams. While cellulose xanthate is very effective for removal of heavy metals from wastewater, the cellulose xanthate becomes loaded with heavy metals periodically and must be disposed of. In accordance with the present invention, it has been found that water soluble tri-thio carbonates, and particularly alkali metal tri-thio carbonates or alkaline earth tri-thio carbonates effectively precipitate heavy metals from wastewaters leaving a substantially non-polluted solution or effluent capable of plant recycle or legal discharge.